Stainless, obviously you don't run the MLS (multiple layer steel) gaskets, cause if you try to anneal them you WILL have leaks................. Bob
p.s. On my Early Olds race motor we run Copper with O-rings on the heads only, and have had good luck with them. Ibelieve that I got them from CopperHead, and sent them a stock head gasket for a patern with the bore size increased. I anneal them every time the engine is appart.